Versatile Asymmetric Separator with Dendrite‐Free Alloy Anode Enables High‐Performance Li–S Batteries

Abstract Lithium–sulfur batteries (LSBs) with extremely‐high theoretical energy density (2600 Wh kg−1) are deemed to be the most likely energy storage system to be commercialized. However, the polysulfides shuttling and lithium (Li) metal anode failure in LSBs limit its further commercialization. Herein, a versatile asymmetric separator and a Li‐rich lithium–magnesium (Li–Mg) alloy anode are applied in LSBs. The asymmetric separator is consisted of lithiated‐sulfonated porous organic polymer (SPOP‐Li) and Li6.75La3Zr1.75Nb0.25O12 (LLZNO) layers toward the cathode and anode, respectively. SPOP‐Li serves as a polysulfides barrier and Li‐ion conductor, while the LLZNO functions as an “ion redistributor”. Combining with a stable Li–Mg alloy anode, the symmetric cell achieves 5300 h of Li stripping/plating and the modified LSBs exhibit a long lifespan with an ultralow fading rate of 0.03% per cycle for over 1000 cycles at 5 C. Impressively, even under a high‐sulfur‐loading (6.1 mg cm−2), an area capacity of 4.34 mAh cm−2 after 100 cycles can still be maintained, demonstrating high potential for practical application.

S-3 side of the pp separator is coated with LLZNO. Specifically, 90 mg of LLZNO and 10 mg of PEO were added in NMP and stirred for 6 h to form a homogeneous slurry. Then the dispersion slurry was coated on the other side of the above PP separator with SPOP-Li coating. After drying and then punched into 19 mm disks stored in Argon glovebox for further use. The carbon nanotubes (CNTs) coated PP separator is prepared regarding the above method.

Sulfur/GO cathode preparation
Commercial sublimed sulfur powders (S) and graphene oxide (GO) (3:1 w/w) were mixed by ball milling for 12h at 250rpm, followed by encapsulating S in GO under 155 °C argon atmosphere for 24 h. Subsequently, S/GO composite, PVDF, and carbon black (8:1:1 w/w/w) were mixed in NMP and stirred for 6 h to make a homogeneous slurry. The S/GO cathodes with a sulfur areal loading of 1.0-1.3 mg cm -2 were obtained by coating on the carbon-coated aluminum foil and vacuum drying for 12h. The highloading S/GO cathodes (> 5.0 mg cm -2 ) were prepared by using an aqueous-based binder of carboxymethylcellulose sodium (CMC) and styrene butadiene rubber (SBR) dispersion solution (48w%) instead of PVDF. Firstly, 22 mg of CMC was added in 2ml deionized water and vigorously stirred for 60 min to completely dissolve, and then 43 mg of carbon black was slowly added with stirring, followed by stirring for another 90 min. after that, 988 mg of S/GO active material was added and stirring was continued for 120 min. Finally, 56 mg of SBR was added and stirred for 30 min to obtain a homogeneous slurry. The same as the above procedure, the homogeneous slurry was coated on carbon-coated Al foil by using a doctor blade and dried in a vacuum oven at 60 °C overnight.

Materials Characterizations
The morphologies and element distribution mappings of materials were obtained by using scanning electron microscopy (SEM, JEOL JSM-7800F). The X-ray diffraction (XRD, SmartLab) of LLZNO was performed from 10-70 o with Cu-Kα radiation (10 o min -1 ). Fourier-transform infrared (FTIR) spectra was measured by a BRUKER S-4 ALPHA spectrometer (4000-400 cm -1 , resolution 2 cm -1 ). The surface area and pore size were collected with Quantachrome autosorb IQ3 system. The surface chemical composition was carried out with X-ray photoelectron spectroscopy (XPS, VG Scientific ESCALAB 2201XL).

Li + ionic conductivity
The blocking stainless steel (SS)/separator-electrolyte/SS symmetric cells were assembled to conduct the electrochemical impedance spectroscopy (EIS) analysis (1-10 5 Hz, 10 mV) by the electrochemical station (CHI 760E, Chenhua) to obtain the impedance of the different separators from 25 to 105 °C. The ionic conductivity(σ) was calculated by the equation (S1): Where l and A are the thickness of the separator and the contacted area, respectively.

Li-ion transference number
The transference number of Li + was estimated by the chronoamperometry test. The symmetric Li/Li cells were assembled with PP separator or modified separator on an electrochemical working station. The initial resistance (R0) was measured by electrochemical impedance spectroscopy (EIS). The initial current (I0) and the steadystate current (Is) can be acquired after 1000 s' chronoamperometric measurement with a constant potential step difference of 10 mV (∆V). After the chronoamperometry measurement, the steady-state resistance (Rs) was measured Li-ion transference number (tLi+) can be calculated by the Equation (S2):

Computational details
The density functional theory (DFT) calculations, which were implemented in the Vienna ab initio simulation package (VASP) code, [1] wasapplied to calculate the binding energies of S 6and Li 2 S 6 clusters. The projector augmented wave (PAW) method was applied to describe the electron-ion interaction. [2] And the electron exchange correlation was represented by the functional of Perdew, Burke and Ernzerhof (PBE) of generalized gradient approximation (GGA). [3] For all the calculations, the cutoff energy was set to be 500 eV. All periodic slab calculations were carried out using a vacuum spacing of at least 15 Å to avoid the interaction between the neighboring periodic structures. The convergence tolerance was 10 -5 eV and 0.01 eV/Å for energy and force, respectively.
The Brillouin zone integrations were performed by using 1×1×3 Monkhorst-Pack for carbon nanotube and 2×2×1 Monkhorst-Pack for SPOP-Li. In addition, DFT-D3 calculations were adopted to describe the van der Waals (vdW) interaction. [4] The    Figure S11. Photographs of the color changes of the Li2S6 solution exposed to different materials.     Table S4. The slope data of the fitting curves (Figure S19 c, f, i, l and o).